1. Field of the Invention
The present invention relates to a sheet feeding apparatus used with an image forming apparatus such as a copying machine, a laser beam printer, an ink jet printer and a facsimile, and other sheet using apparatus.
And more particularly, it relates to an automatic sheet feeding apparatus in which sheets stacked on a sheet stacking plate as a sheet stack are separated and supplied one by one by a separation member and a separation/feed means comprised of a feed member urged against the sheet stack with predetermined pressure.
The sheet may be, for example, a transfer sheet, a recording sheet, a print sheet, an OHP sheet, an original, an envelope, a post card, a card or a film. The material of the sheet is not limited to paper but may be plastic, metal or cloth.
2. Related Background Art
An example of a conventional automatic sheet feeding apparatus is shown in FIGS. 20A and 20B. FIG. 20A is a schematic sectional view of a sheet feed portion of the apparatus and FIG. 20B is a schematic perspective view of the sheet feed portion of the apparatus.
A sheet stacking plate 51, 51a on which sheets S are stacked extends forwardly and downwardly. A front side portion 51 (referred to as "sheet feed pressure plate" hereinafter) of the sheet stacking plate can be pivoted around a hinge portion 51b and is always biased upwardly by a spring member (not shown). In a waiting condition for paper feeding, the sheet feed pressure plate 51 is lowered to a lower waiting position by a hold-down cam (not shown) in opposition to the spring member.
A semi-circular sheet feed roller 52 is disposed above a tip end of the sheet feed pressure plate 51a and is secured to a sheet feed shaft 53. The sheet feed shaft 53 is rotatably supported by bearings between side plates (not shown) of the apparatus and is intermittently rotated by one rotation by means of a one rotation clutch (not shown) in a sheet feeding direction whenever a sheet feed start signal is emitted. Thus, the sheet feed roller 52 is also intermittently rotated by one rotation.
A pair of idler rollers 54 disposed on both sides of the sheet feed roller 52 are rotatably mounted on shafts 55 held by the side plate of the apparatus.
Upper and lower sheet guide plates 56 serve to guide the sheet to a pair of sheet convey rollers 58 disposed at a downstream of the sheet feed roller 52 in a sheet conveying direction.
A separation pad 57 is provided on an upper surface of the lower guide plate 56 at an end thereof near the sheet feed pressure plate. The end of the lower guide plate near the sheet feed pressure plate is always biased upwardly by spring members (not shown) so that the separation pad 57 is urged against the pair of idler rollers 54 with predetermined pressure. During the rotation of the sheet feed roller 52, a cylindrical portion of the sheet feed roller is also urged against the separation pad 57 with predetermined pressure.
1) FIG. 20A shows a waiting condition for paper (sheet) feeding. In this condition, the sheet feed pressure plate 51 is held at a predetermined lower waiting position by hold-down cams (not shown) in opposition to the spring members, and the sheet feed roller 52 is stopped so that a D-cut portion (cutted flat portion) thereof is facing downwardly so as to separate the sheet feed roller 52 from the sheet stack S. PA1 2) In this sheet feeding waiting condition, a sheet feed start signal is inputted to a control circuit, one rotation of the sheet feed shaft 53 and accordingly the sheet feed roller 52 is started. Further, the sheet feed pressure plate 51 is released from the cams, with the result that the sheet feed pressure plate 51 is lifted by the spring members. Rotation of the pair of sheet convey rollers 58 is also started. Incidentally, the pair of sheet convey rollers 58 may be always rotated. PA1 3) A tip end of the fed-out uppermost sheet S1 enters into and passes through a nip (separation nip) between the idler rollers 54 and the separation pad 57 and then is pinched between the cylindrical portion of the rotating sheet feed roller 52 and the separation pad 57 to be supplied by the contact friction of the sheet feed roller 52. This condition is shown in FIG. 21B. The idler rollers 54 are rotatingly driven by the movement of the sheet S1. PA1 4) FIG. 21C shows a condition the sheet S1 is further fed by the rotation of the sheet feed roller 52. Up to this time, the tip end of the sheet reaches a nip between the sheet convey rollers 58 to stably relay the sheet to the pair of sheet convey rollers 58, and the sheet feed pressure plate 51 is again held at the predetermined lower waiting position by the hold-down cams (not shown) in opposition to the spring members to separate the sheet feed roller 52 from the sheet stack S. PA1 5) After the sheet feed roller 52 is rotated by one rotation, when it is returned to the lower waiting position where the cutted flat portion of the roller is facing downwardly, the sheet feed roller is stopped. This condition is shown in FIG. 22A. Even when the sheet feed roller 52 is stopped, the uppermost sheet S1 continues to be conveyed by the pair of sheet convey rollers 58. In the condition that the semi-circular sheet feed roller 52 is separated from the separation pad 57, if the second and other sheets are moved together with the uppermost sheet S1, since the idler rollers 54 are frictionally contacted with the separation pad, only the uppermost sheet S1 is conveyed, and the second and other sheets are prevented from being double-fed. PA1 6) After a trail end of the uppermost sheet S1 being conveyed leaves the nip between the separation pad 57 and the idler rollers 54 and the sheet feed roller 52, when a next sheet feed start signal is inputted to the control circuit, the same operation cycle (from 1 to 5) is repeated to effect the separation and feeding of the next sheet.
In an initial phase of the rotation of the sheet feed roller 52, an edge 52a between the cutted flat portion and the cylindrical portion of the sheet feed roller 52 is urged, with predetermined pressure, against a front upper end portion of the sheet stack S lifted by the sheet feed pressure plate, with the result that a feeding force due to contact friction of the sheet feed roller 52 acts on an uppermost sheet S1 in the sheet stack S. FIG. 21A shows this condition.
Second and other sheets trying to move together with the uppermost sheet S1 are prevented by the separation pad 57 from entering into the nip between the separation pad 57 and the idler rollers 54 and the nip between the separation pad 57 and the cylindrical portion of the sheet feed roller 52, with the result that only the uppermost sheet S1 is supplied by the contact friction of the sheet feed roller 52.
By the way, in the above-mentioned conventional sheet feeding apparatus, in the initial period of the sheet feeding, it is apprehended that the tip end of the sheet S1 does not enter into but is stopped by the nip (separation nip) between the idler rollers 54 and the separation pad 57 to cause the buckling at as tip end portion Sa of the sheet, as shown in FIG. 22B, thereby shrinking or folding or (in the worst case) damaging the tip end of the sheet.
Regarding this, in the initial period of the sheet feeding, as shown in FIG. 21A, the edge 52a between the cutted flat portion and the cylindrical portion of the sheet feed roller 52 contacts with the sheet stack S not at the tip end thereof but at a position spaced apart from the tip end by a distance Sa, and a tip end portion of the sheet S1 (fed out by the sheet feed roller 52) having a length corresponding to this distance Sa is conveyed to the separation nip forwardly of the edge 52a between the cutted flat portion and the cylindrical portion of the sheet feed roller 52 without being backed-up by the sheet feed roller 52 to try to enter into the separation nip between the idler rollers 54 and the separation pad 57 by utilizing resiliency of the sheet itself.
In this case, since the idler rollers can be freely rotated in opposition to the contact friction between these rollers and the separation pad 57 and do not have self-rotational driving force in the sheet conveying direction, in dependence upon the resiliency of the sheet, the resiliency of the sheet does not overcome the resistance force of the separation nip tending to prevent the tip end of the sheet from entering into the separation nip, with the result that the tip end portion Sa of the sheet S1 cannot enter into the separation nip but is stopped by the separation nip. In this condition, since the sheet S1 is further conveyed by the sheet feed roller 52, the buckling of the tip end portion Sa is generated at the separation nip.